An example of a conventional cylinder servo motor is shown in FIG. 24.
The cylinder servo motor shown in FIG. 24 has been proposed by the inventors of the present invention and so forth, and already filed as a PCT application (Application No. PCT/JP00/01117).
With reference to the figure, reference numeral 1 denotes a servo motor unit which is constructed of a rotor 2 and a stator 3; 4, a ball nut which is connected to the rotor 2 at the side at which a load is applied; and 5, a ball screw which is screwed into the ball nut 4 and which serves as an output shaft that moves in the axial direction thereof. A rotational/linear motion converting mechanism is formed by the ball nut 4 and the ball screw 5. In addition, reference numeral 7 denotes a rotation detector which detects an amount of rotation of the rotor 2 and which includes a sensor unit 7a and a signal processing unit 7b. The signal processing unit 7b includes a rotation detector controlling unit 8, a compensation memory 9, and an adder/subtractor circuit 10. In addition, 6 denotes a control unit for the servo motor unit 1; 11, a length measuring device which measures a displacement of a reflection mirror 11a attached to an end portion of the ball screw 5; 19, an external command device; 12, a control device which serves as a testing device and which includes a first input unit 13, a second input unit 14, a subtractor 15, a control unit 16, a memory 17, and a memory rewriter 18; 100, a first input/output unit which is necessary for transmitting data between the external command device 16 and the control unit 6; and 101, a second input/output unit which is necessary for transmitting data between the control device 12 and the compensation memory 9.
In the cylinder servo motor, when a command is issued from the external command device 19 to the control unit 6 via the first input/output unit 100, the control unit 6 starts to rotate the rotor 2 of the servo motor unit 1.
The rotational motion of the rotor 2 is converted to linear motion by the ball nut 4 and the ball screw 5, so that the ball screw 5 starts to reciprocate.
Before the cylinder servo motor is used (before it is shipped from a factory), compensation data for compensating a positioning error (which relies upon accuracy of the components of the cylinder servo motor, accuracy of the rotation detector, etc.), is obtained, and the obtained data of the positioning error is stored in the compensation memory 9.
More specifically, the reflection mirror 11a is attached to an end portion of the ball screw 5, and the length measuring device 11 is disposed such that the length measuring device 11 opposes the reflection mirror 11a. In addition, the control device 12, which serves as a testing device, is prepared.
Then, the external command device 19 issues a command such that the operation of the cylinder servo motor starts and the ball screw 5 starts to reciprocate. A displacement of the ball screw 5 is measured by the length measuring device 11 using the reflection mirror 11a, and the measurement data is input to the control unit 16 of the control device 12 via the second input unit 14. In addition, angle data obtained by the sensor unit 7a is output to the control unit 6 and to the control unit 16 via the second input/output unit 101 and the first input unit 13. The control unit 16 calculates an estimated position of the end portion of the ball screw 5 by multiplying the detection data obtained from the rotation detector 7 by a ball screw lead. Then, the estimated position of the end portion of the ball screw 5 calculated by the control unit 16 and the measurement data obtained by the length measuring device 11 are transmitted to the subtractor 15. The subtractor 15 calculates the positioning error between the estimated position of the end portion of the ball screw 5 and the position measured by the length measuring device 11. The error calculated by the subtractor 15 is stored in the memory 17 via the control unit 16. Then, after the above-described error measurement is completed, the memory rewriter 18 records the error data stored in the memory 17 in the compensation memory 9 via the second input/output unit 101. Accordingly, before this cylinder servo motor is used (before it is shipped from a factory), compensation data with which the positioning error is compensated for is stored in the compensation memory 9.
In normal operation, the reflection mirror 11a, the length measuring device 11, and the control device 12 are removed. In order that the end portion of the ball screw 5 is accurately positioned, detection data of the rotation detector 7 is compensated using the compensation data stored in the compensation memory 9, and the compensated value is output to the control unit 6. The control unit 6 servo-controls the servo motor unit 1 based on the compensated value.
As described above, according to the known cylinder servo motor, positioning accuracy of the ball screw 5 is measured in advance using the length measuring device 11, and the detection data of the rotation detector 7 is compensated so that accurate positioning can be realized. However, in order to obtain the compensation data, the control device 12 for calculating the compensation data and recording it in the compensation memory 9 is required in addition to the length measuring device 11. Accordingly, every time the cylinder servo motor is disassembled for maintenance and/or repair, the control device 12, which is a testing device, must be reset and the compensation data must be re-calculated.
In addition, in the conventional cylinder servo motor, as well as the first input/output unit 100 for transmitting data between the external command device 16 and the cylinder servo motor, the second input/output unit 101 for transmitting data between the control device 12 and the cylinder servo motor is also necessary.
Besides, in the known cylinder servo motor, reduction of positioning accuracy due to temperature variation, backlash, strain caused by stress, etc., are not taken into account. Thus, when the cylinder servo motor is operated, there is a possibility that errors due to temperature variation, backlash, strain, etc., will occur and positional accuracy will be degraded.
In order to compensate for the errors due to temperature variation, backlash, strain, etc., a thermistor, a speed sensor, a force sensor, etc., may be installed. However, in such a case, electric wires, etc., are also necessary in addition to the sensors, so that costs and the number of processes are increased and the maintainability is degraded. In addition, when the detection data obtained by the above-described sensors are processed by the external command device 19, there may be a problem in that a calculation load on the external command device 19 becomes too high.